Building a Compiler with. JoeQ. Outline of this lecture. Building a compiler: what pieces we need? AKA, how to solve Homework 2

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1 Building a Compiler with JoeQ AKA, how to solve Homework 2 Outline of this lecture Building a compiler: what pieces we need? An effective IR for Java joeq Homework hints

2 How to Build a Compiler 1. Choose (or design) the language 2. Write the parser and AST structures 3. Write the frontend: symbol tables, type checking, IR generation 4. Middle end: machine independent optimizations in middle-end IR a. Loop optimizations b. Dataflow optimizations c. Peepholes 5. Back end: register allocation, block and instruction scheduling 6. Generate assembly code 7. Generate machine code and link with target runtime library How to Build a Compiler 1. Choose (or design) the language 2. Write the parser and AST structures 3. Write the frontend: symbol tables, type checking, IR generation 4. Middle end: machine independent optimizations in middle-end IR a. Loop optimizations b. Dataflow optimizations c. Peepholes 5. Back end: register allocation, block and instruction scheduling 6. Generate assembly code 7. Generate machine code and link with target runtime library

3 How to Build a Compiler for Java 1. Reuse the JDK compiler (javac) for the parser and frontend 2. javac generates JVM bytecode 3. Should we use that for IR? Pros and Cons of JVM Bytecode Pro: it s machine independent Pro: it covers everything Java needs, and nothing more Pro: it s high level, somewhat understandable, and has a lot of tooling Pro (for class purposes): javac does not perform any optimization

4 Cons: This is how JVM Bytecode Looks Like int a, b, c; a = 3; if (a < 2) { c = 5; } else { c = 6; } 0: iconst_3 1: istore_1 2: iload_1 3: iconst_2 4: if_icmpge 14 7: iconst_3 8: istore_2 9: iconst_5 10: istore_3 11: goto 19 14: iconst_3 15: Cons: JVM Bytecode is Stack-Oriented int a, b, c; a = 3; if (a < 2) { c = 5; } else { c = 6; } 0: iconst_3 1: istore_1 2: iload_1 3: iconst_2 4: if_icmpge 14 7: iconst_3 8: istore_2 9: iconst_5 10: istore_3 11: goto 19 14: iconst_3 15: push constant 3 pop into reg 1 push from reg 1 push constant 2 pop 2 operands, cond jump push constant 3 pop into reg 2 push constant 5 pop into reg 3 uncond jump push constant 3 Lots of pushes and pops Moving code around is messy Writing efficient JVM bytecode is hard Also useless: HW uses registers not stack!

5 joeq: a Register-Based IR for Java int a, b, c; a = 3; if (a < 2) { c = 5; } else { c = 6; } BB0 (ENTRY) (in: <none>, out: BB2) BB2 (in: BB0 (ENTRY), out: BB3, BB4) 1 MOVE_I R1 int, IConst: 3 2 IFCMP_I IConst: 3, IConst: 2, GE, BB4 BB4 (in: BB2, out: BB5) 3 MOVE_I R2 int, IConst: 3 4 MOVE_I R3 int, IConst: 6 BB3 (in: BB2, out: BB5) 5 MOVE_I R2 int, IConst: 3 6 MOVE_I R3 int, IConst: 5 7 GOTO BB5 joeq: a Register-Based IR for Java joeq: joe Quads, but also advanced in Japanese ( 上級 ) Quad: a quadruple of (target, opcode, operand1, operand2): ADD_I R1 int, R2 int, IConst: 3 Target: R1 Opcode: ADD_I (signed integer addition) Operand 1: R2 Operand 2: immediate 3 Also known as Three Address Code (chapter 6.2)

6 Properties of joeq Quads Operands can be registers or constants There are infinite registers! All operands can be constants (no need for iconst-like operations) Simplifies constant folding Makes everything else harder What joeq Quads Are There? Moves Arithmetic and conversion ops (UnaryOperator, BinaryOperator) Memory ops (GETFIELD, PUTFIELD, ALOAD, ASTORE) Function calls (INVOKEVIRTUAL, INVOKESPECIAL, INVOKEINTERFACE) Control flow (cond jumps, table switches) Special Java operations (MONITORENTRY/EXIT, INSTANCEOF, etc.) A few weird ones (PEEK, POKE)

7 Exception handling in joeq joeq quads never fail Except for those who are specified to fail: NULL_CHECK (throws NullPointerException) ZERO_CHECK (throws DivisionByZeroException) BOUNDS_CHECK (throws ArrayIndexOutOfBoundsException) ASTORE_CHECK (throws ArrayStoreException) Exceptions, even from the CHECK quads, are never modeled in CFG Don t need to worry in this class What joeq Quads Are There, Really? The Full List: To find out what an Operator does: Check if the name matches a JVM bytecode ( Read the implementation Operator.interpret() Ask in Piazza or at OH

8 Manipulating Quads The Quad class: getoperator() : find what the quad does getoperands() : find inputs to the quads getdefinedregisters() : find where the registers written by the quad NOTE: despite the name, a quad can have more than 2 operands! Manipulating Quads Quads are arranged in a ControlFlowGraph Composed of BasicBlock The joeq loader constructs the CFG for you Iterate quads in BB with a QuadIterator or with a QuadVisitor

9 Object Oriented Compiler Design One Object for each IR element Quads, BasicBlock, One Object for each compiler phase A Parser, a Frontend, an Optimizer, a Backend, etc. One Object for each analysis (eg. DeadCodeElimination, or EnterSSA) Analyze and transform IR, often with visitor pattern Object Oriented Compiler Design One Object for each IR element Quads, BasicBlock, One Object for each compiler phase A Parser, a Frontend, an Optimizer, a Backend, etc. Your goal in HW: write a generic dataflow analyzer, with many pluggable analyses Should work with the example analysis (Liveness, Constant Propagation) One Object for each analysis (eg. DeadCodeElimination, or EnterSSA) Analyze and transform IR, often with visitor pattern Your goal in HW: write dataflow analyses for Faintness and Reaching Definitions

10 Homework Details We provide a skeleton, testing code and two example analyses The handout details how to set up joeq We ll go through that in more details on Friday What parts of joeq can you use? Almost all of joeq is allowed Most of it won t be useful though (eg the bytecode loader, the interpreter, or the runtime emulator) Do not use joeq.compiler.dataflow For this homework, also do not use joeq.compiler.quad.ssa No external libraries, and no Java > 1.5 APIs or syntax Check on myth with Java 1.5 that your code works

11 Why joeq? joeq is not a learning goal for this class Unlike say, Java in 106A, Haskell in 242/240H or Tensorflow in 224N/231N So why not learn LLVM instead? The same concepts apply to any mid-level IR (plus or minus SSA) LLVM is vast and complicated, because C++ is vast and complicated